1. Field of the Invention
The present invention relates to a reversible heat-sensitive recording material in which images are formable and erasable by controlling heat energy.
2. Prior Art
A heat-sensitive recording material generally has a support and a heat-sensitive recording layer which is composed mainly of normally colorless or slightly colored electron donating dye precursor and an electron accepting color developer and formed on the support. The dye precursor and the color developer instantaneously react upon application of heat by a thermal head, a thermal pen, laser beams, or the like, to form an image. Such heat-sensitive recording materials are disclosed in Japanese Patent Publications Nos. 43-4160 and 45-14039.
In the above heat-sensitive recording materials, generally, when an image is once formed, it is impossible to erase the image to restore an image portion to its original state. For recording further information, therefore, it is only possible to make recording in a portion where no image is formed. When the area for heat-sensitive recording is limited, there is therefore caused a problem that information that can be recorded is limited, so that not all of the necessary information can be recorded.
For overcoming the above problem, reversible heat-sensitive recording materials capable of repetitive formation and erasure of images have been proposed in recent years. For example, JP-A-54-119377, JP-A-63-39377 and JP-A-63-41186 disclose heat-sensitive recording materials comprising a matrix resin and an organic low-molecular compound dispersed in the matrix resin. In these heat-sensitive recording materials, however, the transparency thereof is reversibly changed, so that the contrast between an imaged portion and an non-imaged portion is insufficient.
There is also proposed a color-forming and color-erasing reversible heat-sensitive recording material on which color formation and color erasure can be repeated. JP-A-2-188293, JP-A-2-188294 and International Patent Publication WO90/11898 disclose reversible heat-sensitive recording materials comprising an electron-donating dye precursor and a color developing and decloring agent for causing the electron-donating dye precursor to form a color upon heating and erasing the color, i.e., a reversible color developer in the present invention.
JP-A-63-173684 and JP-A-4-247985 disclose a reversible heat-sensitive recording material comprising a combination of an electron-donating dye precursor and an ascorbic acid derivative, or a combination of an electron-donating dye precursor and an organic sulfonic acid compound or the like.
The above reversible heat-sensitive recording materials containing an electron-donating dye precursor have various problems caused by the photodecomposition of the electron donating dye precursor itself. That is, when exposed to light such as sunlight or fluorescent lamp light, a non-image portion decreases in whiteness, and the formed color phase of an image portion deteriorates. Further, there is another problem that when an image portion is erased, a non-erasable residue takes place.
In general heat-sensitive recording materials containing a dye precursor, it is general practice to incorporate an ultraviolet absorbent into a protective layer. In reversible heat-sensitive recording materials, however, it produces almost no effect to incorporate an ultraviolet absorbent. That is because general ultraviolet absorbents which undergo no crosslinking with a binder component in a protective layer have a problem that they are crystallized and bleed out after stored for a long period of time and cannot exhibit the capability of ultraviolet absorption. Particularly, in reversible heat-sensitive recording materials on which printing and erasing are repeated, further, an ultraviolet absorbent gradually shifts out of a protective layer, which causes a problem that the capability of ultraviolet absorption decreases and that the reversible heat-sensitive recording layer decreases in printing performance. Further, when a large amount of an ultraviolet absorbent is incorporated for compensating the crystallization and bleed-out of the absorbent, there are caused problems that an image is debased, and that the strength of a layer decreases during repeated printing and erasing due to an increase in the amount of the melting component. In JP-A-9-207437, further, an attempt is made to improve light resistance by adding an ultraviolet-absorbing polymer. However, this polymer has poor compatibility with other components, and the effect of absorbing ultraviolet light is insufficient.